When a desired transmitted radio signal is generated using a non-linear combining of signals (such as by mixing a signal from a local oscillator operating at a channel frequency and a baseband information signal) and/or non-linear amplification of signals, signals at undesired frequencies may also be generated, which are generally referred to as spurious signals, or more simply, spurs. Types of spurs include intermodulation spurs (IM spurs) and image spurs. One type of IM spur that can be generated is referred to as counter IM, which is characterized as having energy at and around a frequency of fLO−3fM, where fLO is the carrier frequency of the desired signal and fM is the center of the modulated baseband signal. Another type of IM spur is characterized as having energy at and around a frequency of fLO+3fM and is produced through mixing of the desired signal fLO+fM and the image spur fLO−fM. This is referred to as image induced intermodulation (IIIM) in this document.
In some cases, the spurs fall within another assigned channel and exceed desired or approved energy levels. A known method of mitigating this problem is to reduce the transmitted power, but this reduces the range of the desired signal.
An interference problem can arise in some devices which have a processor clock. The processor clock circuitry typically involves an oscillator and square wave generating circuits. The processor clock circuitry can produce radiated energy at harmonics of the frequency of the oscillator or square wave. These harmonics can introduce undesired signals when the device transmits a desired signal. The undesired signal (spurious signal) may have enough energy at a particular channel frequency used by other devices to cause interference with a desired signal that is being transmitted to the other devices. In some designs, the frequency of the oscillator is offset to change the frequency of the undesired signal so that it is no longer on the particular channel frequency.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.